Low voltage AC (alternating current) contactors are used in industrial and commercial applications to control power flow to electrical motor and lighting loads in circuits operating at 600 V RMS and below. High capacity contactors typically have two contact gaps in series per electrical phase, with each contact gap having two contact pads. Such contactors are commonly constructed to include a moveable bridge structure on which two electrically connected silver-based contact pads are positioned. The bridge is driven by a solenoid acting in opposition to a spring such that the bridge contacts can make and break contact, depending on the bridge position, with two corresponding fixed contacts disposed in a plastic enclosure to which the voltage supply and load supply leads are attached. When the bridge is moved such that the bridge contact pads are disposed in contact with the respective stationary contact pads the circuit is closed; to open the circuit the bridge assembly is moved to separate the bridge contact pads from the respective stationary contact pads. Due to the electrical arcing that occurs when the contact pads are moved to break the circuit, many high capacity contactors also have arc chutes in the plastic enclosure that are disposed in proximity to the contact pads to assist in circuit interruption.
In a typical circuit interruption of a single phase, the circuit starts with the contactor contacts closed, current flowing, and the source voltage almost entirely across the electrical load. The bridge is then moved, e.g., by de-energizing the solenoid which allows the spring to move the bridge so that a gap exists between the bridge contact pads and the stationary (or fixed) contact pads. This opening of the contact pads results in low voltage arcs appearing across the two contact gaps (per electrical phase) in series, but otherwise has little immediate effect because the 10 volt to 30 volt drop across the arcs is relatively small compared to the voltage available from the power source, and thus the current continues to flow, from cathode spots on a negative contact pad to anode spots on the corresponding contact pad in the pair, with the amount of current substantially the same as before the contacts were open. The arc currents interact with magnetic blowout structures which tend to push the arc to the arc chute structure; because the arc chute structure presents a high voltage drop, there is a tendency of the arc to remain across, or to return to, the now-open contact pads.
After the opening of the contact pads, ultimately the current drops to zero at some time less than half a period of the AC waveform. Current can begin flowing again, however, if a new arc is formed as the AC waveform goes to a non-zero value and new cathode spots are formed on the former arc anodes by transfer of the available voltage source from the load to the contactor. Such a voltage transfer happens quickly, in about half the period of the resonant (or ringing) frequency of the circuit, which is typically as long as about 50 .mu.sec in a low to medium power circuit (e.g., about 10 KW to less than 100 KW) operating at a low voltage (.ltoreq.600 V RMS) and as short as about 5 .mu.sec in a medium to high power circuit (e.g., between about 100 KW to 1000 KW) operating at a low voltage (in higher power circuits, the impedance tends to be lower). The magnitude of the voltage available to reestablish the arc is determined by the product of the peak source voltage, the sine of the phase angle between the voltage and current at the zero current point, and an overshoot factor near unity. If the magnitude of this available voltage is sufficient, conduction is reestablished along a series of arc gaps through the contactor and the current flow will continue at least for another half-period of the power frequency; otherwise current flow in the circuit remains interrupted.
In high current capacity conventional contactors, arc chutes are disposed in the contactor near the contact pads and the contactor is designed so that magnetic forces tend to force an arc across the contact pads into the arc chute. Due to the higher voltage drops across the several plates disposed in series in the arc chute, current flow is reduced. The limitations of such devices operating in ambient conditions, however, have necessitated the use of the bridge structure with the dual contact pads as described above, as the corresponding two contact gaps in series per phase enable a given size of contactor to handle higher voltage circuits for voltages in the upper half (e.g., .gtoreq.300 V RMS) of the low voltage range.
To assure adequate current interruption capability, standard ratings for contactors have been established. For example, the National Electric Manufacturers Association (NEMA) has established general purpose standard ratings of sizes 00 through 9; these ratings pertain to contactors intended for use in switching service at RMS AC currents ranging from 9 to 2250 amperes and motor power rating from 3/4 horsepower to 1600 horsepower, in single phase or three phase systems at voltages up to 600 volts RMS (as used in this context, "low voltage" is considered to be 600 volts or less). Similar standard ratings have been established by the International Electrotechnical Committee in Europe. Manufacture of contactors to meet the required performance standards, such as current carrying and interrupt capability, for different service voltages has thus far required the use of the dual-contact pad bridge. The dual contact bridge construction has manufacturing costs associated with the use of four silver based contact pads per phase; for example, between about 30% and 50% of the materials cost of the controller arises from the silver based contact pads. Further, the dual contact construction further imposes some operating limitations, such as limited heat dissipation.
It is an object of this invention to provide a reliable low voltage AC contactor having only two contact pads per phase for standard rating that requires four contact pads per phase in a conventional contactor.
It is a further object of this invention to provide a contactor having improved heat dissipation characteristics as compared with a contactor of the same standard rating that requires four contact pads per phase.
It is a still further object of this invention to provide a low voltage AC contactor having a controllable atmosphere in the gap between contact pads to enhance the high voltage recovery value of that atmosphere.